Detergent compositions designed for use in automatic dishwasher machines are well known, and a consistent effort has been made by detergent manufacturers to improve the cleaning and/or rinsing efficiency of said compositions on dishes and glassware, as reflected by numerous patent publications.
The present invention is concerned with the silver-tarnishing problem encountered when detergent compositions which contain oxygen-bleaching species are employed in machine dishwashing methods.
The satisfactory removal of bleachable soils such as tea, fruit juice and coloured vegetable soils, such as carotenoid soils is a particular challenge to the formulator of a machine dishwashing detergent composition. Traditionally, the removal of such soils has been enabled by the use of bleach components such as oxygen and chlorine bleaches.
A problem encountered with the use of such bleaches is the tarnishing of any silverware components of the washload. Oxygen bleaches tend to give rise to the problem of tarnishing more than chlorine bleaches. The level of tarnishing observed can range from slight discolouration of the silverware to the formation of a dense black coating on the surface of the silverware.
The formulator thus faces the dual challenge of formulating a product which maximises bleachable soil cleaning but minimises the occurrence of tarnishing of silverware components of the washload.
The Applicants have found that the problem of tarnishing can be particularly severe when an oxygen bleaching species is employed especially when the formulation has a pH below 9.6. Oxygen bleaching species are however, preferred over chlorine bleaches for reasons of environmental compatibility.
It has been found that enhanced anti-silver tarnishing as well as good cleaning performance can be achieved through control of oxygen-bleaching power, and the inclusion of a paraffin oil, and a heavy metal ion sequestrant into the detergent compositions.
Preferably, the rate of release of the oxygen bleach is also controlled. The rate of release of oxygen bleach is preferably rapid enough to provide satisfactory cleaning, but not so rapid that tarnishing is enabled.
It is an object of the present invention to provide compositions, suitable for use in machine dishwashing methods, having enhanced anti-silver tarnishing properties, as well as good cleaning performance, particularly bleachable soil removal performance.
The present invention also encompasses a making process for the detergent compositions herein, which optimizes the anti silver-tarnishing performance of the resulting compositions.
EPA 150 387 discloses chlorine-bleach based machine dishwashing compositions containing a paraffin wax as suds suppressor. EPA 186 088 discloses machine dishwashing compositions based on carbonates and silicates, and optionally chlorine or oxygen bleaches containing paraffin oils as dust binders.